Disk apparatus to which iterative decoding is applied and method for operating log likelihood ratios in the same

ABSTRACT

According to one embodiment, a method for operating log likelihood ratios in a disk apparatus is disclosed. Iterative decoding is applied to the disk apparatus. The method can set windows for a sequence of the log likelihood ratios output by a soft-decision most-likelihood decoder based on the sequence of the log likelihood ratios or an amplitude of a read signal acquired in response to read of data from a data sector on a disk carried out by a head. The method can multiply the log likelihood ratios contained in each of the windows, by a multiplier specific to each window. In addition, the method can transmit the sequence of the log likelihood ratios multiplied by the multiplier for each of the windows to a parity check decoder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-021427, filed Feb. 2, 2010; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a disk apparatus towhich iterative decoding is applied and a method for operating loglikelihood ratios in the disk apparatus.

BACKGROUND

In some recently developed disk apparatuses to which iterative decodingis applied, a low density parity check (LDPC) code is used as a paritycheck code added to data written to a disk. In these disk apparatuses,each bit of data read from the disk is decoded by an LDPC decoder basedon a log likelihood ratio (LLR) output by a soft-decisionmaximum-likelihood decoder. A soft output Viterbi algorithm (SOVA) isapplied to the soft-decision maximum-likelihood decoder.

LLR is a logarithmic form of the ratio of the probability (likelihood)that the corresponding bit is “1” to the probability that the bit is“0”. When positive, LLR indicates that the corresponding bit is morelikely to be “1”. When negative, LLR indicates that the correspondingbit is more likely to be “0”. When LLR is zero, the corresponding bitcan be determined to be “1” or “0” at the same probability. That is,when LLR is zero, the corresponding bit is least reliable. Thus, LLR isreliability (probability) data indicative of the degree of reliability(probability) at which the corresponding bit is “1” or “0”.

The LDPC decoder carries out parity check based on LDPC code added todata corresponding to LLRs output by the soft-decisionmaximum-likelihood decoder, to update LLRs. Based on updated LLRs, thesoft-decision maximum-likelihood decoder outputs new LLRs. Thus, LLRsare repeatedly propagated between the soft-decision maximum-likelihooddecoder and the LDPC decoder under a predetermined condition. Thepropagation of LLRs is called probability propagation. Data is decodedby iteration of propagation of LLRs, that is, iterative decoding.

It is assumed that LLRs are partly low in a certain portion of the databecause of a defect on a disk. The presence of such a portion may affectthe other, higher LLRs owing to probability propagation.

Thus, for example, Jpn. Pat. Appln. KOKAI Publication No. 2008-112527(hereinafter referred to as the prior art document) discloses atechnique to mask the part of LLRs corresponding to a defective portion(hereinafter referred to as a medium defective portion) on the diskbased on the above-described LLRs or the amplitude of a signal (readsignal) read from the disk by a head. The technique described in theprior art document uses a scaling factor a to reduce the part of LLRscorresponding to the medium defective portion, thus suppressing theadverse effect of propagation of the part of LLRs corresponding to themedium defective portion.

The medium defective portion is roughly classified into a mediumdefective portion associated with a sharp decrease in the amplitude ofthe read signal (this type is hereinafter referred to as a first mediumdefective portion) and a medium defective portion associated with agradual decrease in the amplitude of the read signal (this type ishereinafter referred to as a second medium defective portion). At theboundary of the first medium defective portion, the amplitude of theread signal or LLRs decrease or increase rapidly. Thus, the boundary ofthe first medium defective portion can be accurately detected based onthe amplitude of the read signal (for example, the moving average of theamplitude) or LLRs.

In contrast, it is difficult to accurately detect the boundary of thesecond medium defective portion based on the amplitude of the readsignal or LLRs. Furthermore, even if the boundary of the second mediumdefective portion is successfully detected, data is not alwayssuccessfully read from a data sector with the second medium defectiveportion. Thus, there has been a demand to effectively control theadverse effect of the second medium defective portion on normalportions.

BRIEF DESCRIPTION OF THE DRAWINGS

A general architecture that implements the various feature of theembodiments will now be described with reference to the drawings. Thedrawings and the associated descriptions are provided to illustrate theembodiments and not to limit the scope of the invention.

FIG. 1 is a block diagram showing the exemplary configuration of amagnetic disk drive according to an embodiment;

FIG. 2 is a diagram showing a first example of a defective portionpresent on a data sector, by means of an envelope for a read signal anda log likelihood ratio distribution corresponding to the read signal;

FIG. 3 is a diagram showing a second example of a defective portionpresent on a data sector, by means of an envelope for a read signal anda log likelihood ratio distribution corresponding to the read signal;

FIG. 4 is a diagram showing an example of windows and a log likelihoodratio distribution for the sequence of log likelihood ratioscorresponding to a data sector;

FIG. 5 is a diagram showing an operation multiplier applied to each ofwindows after shipment of a magnetic disk drive, together with a loglikelihood ratio distribution, according to the embodiment;

FIG. 6 is a diagram showing an operation multiplier applied to each ofwindows during a manufacturing process for the magnetic disk drive,together with a log likelihood ratio distribution, according to theembodiment;

FIG. 7 is diagram showing an example of the distribution of the absolutevalues of log likelihood ratios corresponding to a data sector and themoving average of the absolute values of the log likelihood ratios;

FIG. 8 is a flowchart showing the exemplary procedure of window settingapplied to the embodiment;

FIG. 9 is a flowchart showing a modification of the procedure of windowsetting;

FIG. 10 is a diagram illustrating an exemplary method for determining anoperation multiplier applied to each of windows after shipment of amagnetic disk drive according to the embodiment; and

FIG. 11 is a diagram illustrating an exemplary method for determining anoperation multiplier applied to each of windows during the manufacturingprocess for the magnetic disk drive according to the embodiment.

DETAILED DESCRIPTION

Various embodiments will be described hereinafter with reference to theaccompanying drawings. In general, according to one embodiment, a methodfor operating log likelihood ratios in a disk apparatus is disclosed.Iterative decoding is applied to the disk apparatus. The method can setwindows for a sequence of the log likelihood ratios output by asoft-decision most-likelihood decoder based on the sequence of the loglikelihood ratios or an amplitude of a read signal acquired in responseto read of data from a data sector on a disk carried out by a head. Themethod can multiply the log likelihood ratios contained in each of thewindows, by a multiplier specific to each window. In addition, themethod can transmit the sequence of the log likelihood ratios multipliedby the multiplier for each of the windows to a parity check decoder.

FIG. 1 is a block diagram showing the configuration of a magnetic diskdrive (HDD) according to an embodiment. In FIG. 1, a disk (magneticdisk) is spun at a high speed by a spindle motor (not shown in thedrawings). A head 12 is located over a recording surface of the disk 11.The head 12 is used to write and read data to and from the disk 11. Thehead 12 is attached to the leading end of an actuator 13. The head 12flies over the disk as a result of high-speed spinning of the disk 11.The actuator 13 is driven by a voice coil motor (not shown in thedrawings) to move the head 12 in the radial direction of the disk 11.

In the embodiment, a low density parity check (LDPC) code is added, as aparity check code, to data written to a data sector on the disk by thehead 12. The data written to the data sector (more specifically, thetarget data sector) on the disk 11 is read by the head 12. Then, thehead 12 outputs a read signal. The read signal is amplified by apreamplifier 14.

The read signal amplified by the preamplifier 14 is transferred to ananalog-to-digital converter (ADC) 16 via an analog front-end (analogfront-end circuit) 15. The analog front-end 15 includes a variable gainamplifier (VGA) and an analog filter. The analog-to-digital converter 16converts the read signal into a sequence of digital data at apredetermined sampling period. The sequence of digital data output bythe analog-to-digital converter 16 is equalized into a target partialresponse (PR) waveform by, for example, a finite impulse response (FIR)filter 17 serving as a digital equalizer.

The sequence of the digital data (more specifically, code data)equalized by the FIR filter 17 (PR equalization) is input to asoft-decision maximum-likelihood decoder 18. Based on a soft outputViterbi algorithm (SOVA), the soft-decision maximum-likelihood decoder18 outputs a sequence of log likelihood ratios (LLRs) from the sequenceof the digital data. As described above, when positive, each of LLRsindicates that the corresponding bit is more likely to be “1”. Whennegative, each of LLRs indicates that the corresponding bit is morelikely to be “0”. When each of LLRs is zero, the corresponding bit canbe determined to be “1” or “0” at the same probability. The maximumvalue of each positive LLR is expressed as +LLRmax. One of negative LLRswith the maximum absolute value is expressed as −LLRmax.

The sequence of LLRs output by the soft-decision most-likelihood decoder18 is input to an LLR controller 21. The LLR controller 21 normallyoutputs the sequence of LLRs output by the soft-decision most-likelihooddecoder 18, to an LDPC decoder 19 without performing any specialoperation on the sequence. That is, the sequence of LLRs output by thesoft-decision most-likelihood decoder 18 is transferred to the LDPCdecoder 19 through the LLR controller 21.

Furthermore, in an LLR operation mode, the LLR controller 21 performs aproduct operation (multiplication) on the first sequence of LLRs outputby the soft-decision most-likelihood decoder 18 in response to read ofdata from the data sector carried out by the head 12. In the embodiment,as described later, LLRs contained in each of windows set by a defectdetector 20 are multiplied by a multiplier (or a multiplier factor)specific to the window. The sequence of LLRs multiplied by themultiplier (hereinafter referred to as the operation multiplier) istransferred to the LDPC decoder 19. The setting of the LLR operationmode will be described below.

The LDPC decoder 19 carries out parity check based on the LDPC codeadded to the data corresponding to the sequence of LLRs transferred bythe LLR controller 21. The LDPC decoder 19 thus updates each LLR in thesequence. The LDPC decoder 19 outputs the sequence of the updated LLRsto the soft-decision maximum-likelihood decoder 18.

With the sequence of LLRs directly or indirectly propagated between thesoft-decision most-likelihood decoder 18 and the LDPC decoder 19 asdescribed above, the operation is reiterated under a predeterminedcondition. Based on each LLR in the sequence of LLRs resulting fromcompletion of the iterative operations, the LDPC decoder 19 decodes asequence of hard decision values, that is, a sequence of binary data.The LDPC decoder 19 carries out parity check on all the codes for thedecoded binary data. If as a result of the parity check, the parity issatisfied for all the codes, this means that the decoding has finishednormally. On the other hand, if as a result of the parity check, any oneof the codes fails to satisfy the parity, a read error has occurred. Inthis case, a retry to read data from the target data sector (that is, aread retry) is carried out.

The read retry is carried out up to a predetermined number of times Nuntil the data read succeeds. In the embodiment, given that M is anatural number less than N, for example, after shipment of HDD, if thedata read fails consecutively even with M iterated read retries, CPU 22described below sets an LLR operation mode. The LLR operation mode isalso set if during the process of manufacturing HDD, more specifically,during the process of detecting a defective sector on the disk 11, adefective portion on any data sector is detected by the defect detector20.

Here, it is assumed that a defective portion (medium defective portion)is present on the target data sector. In this case, LLRs correspondingto the defective portion are propagated between the soft-decisionmost-likelihood decoder 18 and the LDPC decoder 19. This may affect LLRscorresponding to the other, normal portions. FIGS. 2 and 3 show a firstand a second examples of defective portions present on the data sector,by means of an envelope for the read signal and an LLR distributioncorresponding to the read signal.

In FIG. 2, reference number 210 denotes an envelope of the read signalobtained when data is read from the data sector by the head 12.Reference number 220 denotes an LLR distribution for the LLR sequencecorresponding to the read signal. The envelope 210 for the read signalshows an envelope 211 corresponding to the positive maximum amplitude ofthe read signal and an envelope 212 corresponding to the negativemaximum amplitude of the read signal. The x axis for the LLRdistribution 220 indicates a bit number (sample point number) BN. The yaxis for the LLR distribution 220 indicates the value of LLR. Arrows 214and 215 on the LLR distribution 220 indicate that LLRs corresponding tothe defective portion 213 affect LLRs corresponding to other, normalportions as a result of propagation.

In the example in FIG. 2, the amplitude of the read signal decreasessharply at the boundary of the defective portion 213 enclosed by anellipse. That is, the defective portion 213 corresponds to theabove-described first medium defective portion. In the LLR distribution220, the values of LLRs corresponding to the defective portion 213concentrate in the vicinity of zero (0) unlike in the case of LLRscorresponding to the other, normal portions. In this case, a readsuccess rate is low in the LDPC decoder 19. Thus, this type of datasector is likely to be detected to be defective during the process ofmanufacturing HDD, and is thus nonproblematic.

Furthermore, the amplitude of the read signal or LLRs increase ordecrease rapidly at the boundaries Px and Py of the defective portion213. Thus, even by a well-known technique for detecting a defectiveportion based on the amplitude of the read signal (for example, themoving average of the amplitude) or LLRs, the boundaries Px and Py ofthe defective portion 213 can be accurately detected. Thus, a window formasking LLRs present between Px and Py can be set.

In FIG. 3, reference number 310 denotes an envelope for the read signalobtained when data is read from the data sector by the head 12.Reference number 320 denotes an LLR distribution for the LLR sequencecorresponding to the read signal. The envelope 310 for the read signalshows an envelope 311 corresponding to the positive maximum amplitude ofthe read signal and an envelope 312 corresponding to the negativemaximum amplitude of the read signal. Arrows 314 and 315 indicate thatLLRs corresponding to a defective portion 313 affect LLRs correspondingto other, normal portions as a result of propagation.

In the example in FIG. 3, the amplitude of the read signal decreasesgradually at the boundary of the defective portion 313 enclosed by anellipse. In the LLR distribution, the values of LLRs corresponding tothe defective portion 313 vary widely between 0 and the level of LLRscorresponding to the other, normal portions. In such an example as shownin FIG. 3, it is difficult to detect the boundary of the defectiveportion 313 based on the amplitude of the read signal (for example, themoving average of the amplitude) or LLRs. For example, a pair of Px1 andPy1, a pair of Px2 and Py2, or a pair of Px3 and Py3 shown in FIG. 3 maybe detected to be the boundary of the defective portion 313. If theboundary of the defective portion 313 is incorrectly detected, LLRs maybe masked even for bits with effective LLRs or LLRs to be masked mayfail to be masked. In such a case, the rate of bit errors BER in datareads from the data sector with the defective portion 313 may increase.

Furthermore, even if the boundary of the defective portion 313 issuccessfully detected, read of data from the data sector with thedefective portion 313 is not always successful. Moreover, the readsuccess rate for the data sector with the defective portion 313 islikely to be affected by a small variation in amplitude caused by avariation in write quality and temperature environment. Hence, the readsuccess rate varies significantly depending on the depth of thedefective portion 313. Thus, the data sector with the defective portion313 is likely to fail to be detected to be defective during themanufacturing process.

Thus, HDD according to the embodiment is configured to be capable ofpreventing LLRs corresponding to the defective portion 313 of the typeshown in FIG. 3 from affecting LLRs corresponding to the other, normalportions after shipment of HDD. This configuration enables the datasector with the defective portion 313 to be detected to be defectiveduring the process of manufacturing HDD as described below.

The embodiment will be described in further detail with reference againto FIG. 1. The defect detector 20 detects a defective portion (mediumdefective portion) on a data sector on the disk 11 based on an outputfrom the preamplifier 14, an output from the analog front-end 15, anoutput from the analog-to-digital converter 16, an output from the FIRfilter 17, or the first output from the soft-decision most-likelihooddecoder 18; all the outputs are provided in response to the read of datafrom the data sector carried out by the head 12. Here, if the outputfrom the preamplifier 14 or the output from the analog front-end 15 isused to detect the defective portion, the defect detector 20 need tocomprise an analog-to-digital converter configured to convert the outputfrom the preamplifier 14 or the output from the analog front-end 15 intodigital data at a predetermined sampling period.

The defect detector 20 comprises a memory 210. The memory 210 is used totemporarily store a sequence of digital data output by theanalog-to-digital converter provided in the defect detector 20, asequence of digital data output by the analog-to-digital converter 16, asequence of digital data output by the FIR filter 17 and having anequalized waveform, or the first sequence of LLRs output by thesoft-decision most-likelihood decoder 18, in response to the read ofdata from a data sector.

The defect detector 20 classifies the sequence of digital data orsequence of LLRs stored in the memory 210 into levels set based on atleast one threshold. Based on this classification (that is, the levelclassifying), the defect detector 20 sets the windows at correspondingdifferent positions in the data sector in association with the sequenceof LLRs. Here, the sequence of digital data corresponds to the amplitudeof the read signal. That is, the defect detector 20 sets the windowsbased on the amplitude of the read signal or the sequence of LLRs andthe at least one threshold. Here, the window corresponding to the lowestlevel corresponds to the defective portion of the data sector. Thewindow corresponding to the highest level corresponds to the normalportion of the data sector. The defect detector 20 also determines, foreach window, the operation multiplier to be applied to the productoperation for LLRs output by the soft-decision most-likelihood decoder18 and contained in each of the windows.

In the LLR operation mode, the LLR controller 21 multiplies the firstLLRs output by the soft-decision most-likelihood decoder 18 andcontained in each of the windows set by the defect detector 20, by theoperation multiplier which is specific to the window and which isdetermined by the defect detector 20. The LLR controller 21 transfersthe sequence of LLRs multiplied by the multiplier for each of thewindows to the LDPC decoder 19.

CPU 22 functions as a main controller configured to control HDD as awhole. In particular, in the embodiment, CPU 22 functions as aninterface between the defect detector 20 and the LLR controller 21. Thatis, CPU 22 instructs the LLR controller 21 to perform the productoperation using the multiplier which is specific to each of the windowsand which is determined by the defect detector 20 in accordance with thesetting of the windows carried out by the defect detector 20. CPU 22 maybe allowed to carry out at least one of the defect detection, windowsetting, and determination of the operation multiplier for each window.

Now, with reference to FIGS. 4 to 6, operations will be described whichare performed by the defect detector 20 and the LLR controller 21 whenthe defect detector 20 sets seven windows. FIG. 4 shows an example ofwindows and an LLR distribution for a sequence of LLRs corresponding toa data sector. In FIG. 4, seven windows W1, W2 _(L), W2 _(R), W3 _(L),W3 _(R), W4 _(L), and W4 _(R) are set by the defect detector 20 based onthe sequence of LLRs corresponding to the data sector. In the example inFIG. 4, the sequence of LLRs corresponding to the data sector isclassified into four levels.

Window (first window) W1 corresponds to the lowest level. Windows(second windows) W2 _(L) and W2 _(R) correspond to the highest level.That is, window W1 corresponds to a defective portion of the datasector. Windows W2 _(L) and W2 _(R) correspond to normal portions of thedata sector. Windows (third windows) W3 _(L) and W3 _(R) correspond to alevel higher than and next to the lowest one. Windows (fourth windows)W4 _(L) and W4 _(R) correspond to a level lower than and next to thehighest one. That is, windows W3 _(L) and W3 _(R) correspond to adefective portion side of the boundary (transition) portion between thedefective portion and the normal portion and are set closer to windowW1. On the other hand, windows W4 _(L) and W4 _(R) correspond to anormal portion side of the boundary portion between the defectiveportion and the normal portion and are set closer to windows W2 _(L) andW2 _(R), respectively.

FIGS. 5 and 6 show an operation multiplier applied to each of thewindows in the example of the LLR distribution and windows shown in FIG.4. FIGS. 5 and 6 also show the LLR distribution. FIG. 5 shows an exampleof an operation multiplier applied to each of the windows after shipmentof HDD according to the embodiment. FIG. 6 shows an example of anoperation multiplier applied to each of the windows during the processof manufacturing HDD.

LLR operation applied to each of the windows after shipment of HDDaccording to the embodiment will be described with reference to FIG. 5.As shown in FIG. 5, the LLR controller 21 multiplies each of LLRs inwindow W1, corresponding to the lowest level (defective portion 510), bya predetermined operation multiplier (minimum multiplier or firstmultiplier) less than 1, for example, 0.5. Thus, the values of LLRscorresponding to the defective portion 510 are changed to directionsshown by arrows 518 and 519 in FIG. 5, that is, to the directions inwhich the absolute values of LLRs decrease.

Furthermore, the LLR controller 21 multiplies each of LLRs in windows W2_(L) and W2 _(R), corresponding to the highest level (normal portion),by a predetermined operation multiplier (maximum multiplier or secondmultiplier) greater than 1, for example, 2.0. Thus, the values of LLRscorresponding to the normal portion are changed to directions shown byarrows 514 to 517 in FIG. 5, that is, to the directions in which theabsolute values increase.

Thus, after shipment of HDD, the values of LLRs corresponding to thedefective portion 510 are reduced, whereas the values of LLRscorresponding to the normal portion are increased. This relativelyreduces the adverse effect of LLRs corresponding to the defectiveportion 510 on LLRs corresponding to the normal portion as shown byarrows 511 and 512 in FIG. 5. As a result, the bit error rate BER forthe read of data from the data sector with the defective portion 510 canbe improved.

Furthermore, the LLR controller 21 multiplies each of LLRs in window W3_(L) corresponding to the defective portion side of the boundary 513between the defective portion 510 and the normal portion, by anoperation multiplier (third multiplier) less than 1 and greater than orequal to the minimum multiplier (0.5), for example, 0.7. Thus, thevalues of LLRs corresponding to the defective portion side of theboundary 513 between the defective portion 510 and the normal portionare changed to a direction shown by arrow 520 in FIG. 5, that is, thedirection in which the absolute values decrease. Although not shown inFIG. 5, the LLR controller 21 also multiplies each of LLRs in window W3_(R) by, for example, 0.7.

Furthermore, the LLR controller 21 multiplies each of LLRs in window W4Lcorresponding to the normal portion side of the boundary 513 between thedefective portion 510 and the normal portion, by an operation multiplier(fourth multiplier) greater than 1 and less than or equal to the maximummultiplier (2.0), for example, 1.2. Thus, the values of LLRscorresponding to the normal portion side of the boundary 513 between thedefective portion 510 and the normal portion are changed to a directionshown by arrow 519 in FIG. 5, that is, the direction in which theabsolute values increase. Although not shown in FIG. 5, the LLRcontroller 21 also multiplies each of LLRs in window W4 _(R) by, forexample, 1.2.

As described above, after shipment of HDD, the LLR controller 21 reducesthe values of not only LLRs corresponding to the defective portion 510but also LLRs corresponding to the defective portion side of theboundary between the defective portion 510 and the normal portion, whileincreasing the values of not only LLRs corresponding to the normalportion but also LLRs corresponding to the normal portion side of theboundary. Thus, the adverse effect of LLRs corresponding to thedefective portion 510 on LLRs corresponding to the normal portion can berelatively reduced.

LLR operation applied to each of the windows during the process ofmanufacturing HDD according to the embodiment will be described withreference to FIG. 6. As shown in FIG. 6, the LLR controller 21multiplies each of LLRs in window W1 corresponding to the lowest level(defective portion) by a predetermined operation multiplier (maximummultiplier or first multiplier) greater than 1, for example, 2.0. Thus,the values of LLRs corresponding to the defective portion are changed todirections shown by arrows 618 and 619 in FIG. 6, that is, thedirections in which the absolute values increase.

Furthermore, the LLR controller 21 multiplies each of LLRs in windows W2_(L) and W2 _(R) corresponding to the highest level (normal portion) bya predetermined operation multiplier (minimum multiplier or secondmultiplier) less than 1, for example, 0.5. Thus, the values of LLRscorresponding to the normal portion are changed to directions shown byarrows 614 to 617 in FIG. 6, that is, the directions in which theabsolute values decrease.

As described above, during the process of manufacturing HDD, the LLRcontroller 21 increases the values of LLRs corresponding to thedefective portion, while reducing the values of LLRs corresponding tothe normal portion. Thus, the adverse effect of LLRs corresponding tothe defective portion on LLRs corresponding to the normal portion asshown by arrows 611 and 612 in FIG. 6 can be relatively increased. As aresult, the bit error rate BER for read of data from a data sector withsuch a defective portion can be deteriorated, enabling the data sectorwith the defective portion to be reliably detected to be defective.

Furthermore, the LLR controller 21 multiplies each of LLRs in window W3_(L) corresponding to the defective portion side of the boundary betweenthe defective portion and the normal portion, by an operation multiplier(third multiplier) greater than 1 and less than or equal to the maximummultiplier (2.0), for example, 1.4. Thus, the values of LLRscorresponding to the defective portion side of the boundary between thedefective portion and the normal portion are changed to a directionshown by arrow 620 in FIG. 6, that is, the direction in which theabsolute values increase. Although not shown in FIG. 6, the LLRcontroller 21 also multiplies each of LLRs in window W3 _(R) by, forexample, 1.4.

Furthermore, the LLR controller 21 multiplies each of LLRs in window W4_(L) corresponding to the normal portion side of the boundary betweenthe defective portion and the normal portion, by an operation multiplier(fourth multiplier) less than 1 and greater than or equal to the minimummultiplier (0.5), for example, 0.8. Thus, the values of LLRscorresponding to the normal portion side of the boundary between thedefective portion and the normal portion are changed to a directionshown by arrow 619 in FIG. 6, that is, the direction in which theabsolute values decrease. Although not shown in FIG. 6, the LLRcontroller 21 also multiplies each of LLRs in window W4 _(R) by, forexample, 0.8.

As described above, during the process of manufacturing HDD, the LLRcontroller 21 increases the values of not only LLRs corresponding to thedefective portion but also LLRs corresponding to the defective portionside of the boundary between the defective portion and the normalportion, while decreasing the values of not only LLRs corresponding tothe normal portion but also LLRs corresponding to the normal portionside of the boundary. Thus, the adverse effect of LLRs corresponding tothe defective portion on LLRs corresponding to the normal portion can berelatively increased. As a result, the data sector with the defectiveportion can be more reliably detected to be defective.

Thus, according to the embodiment, the sequence of LLRs is classifiedinto a plurality of levels based on or the sequence of LLRs or theamplitude of a read signal acquired in response to read of data from thedata sector on the disk carried out by the head. Then, the windowscorresponding to the respective levels are set for the sequence of LLRs.The product operation using the operation multiplier for each window isperformed on LLRs contained in each of the set windows. Thus, the effectof LLRs corresponding to the defective portion on the data sector onLLRs corresponding to the normal portion can be effectively controlled.

Now, the procedure of window setting applied according to the embodimentwill be described with reference to FIGS. 7 and 8. In this case, by wayof example, windows are set by classifying the sequence of LLRs (theabsolute values of LLRs) corresponding to the data sector into levelsset by two thresholds TH1 and TH2 (TH1>TH2). Here, for simplification ofdescription, it is assumed that three types of windows are set inassociation with three levels. FIG. 7 shows an example of thedistribution of the absolute values (|LLR|) of LLRs corresponding to onedata sector and a moving average line 700 for the absolute values ofLLRs. FIG. 8 is a flowchart showing the procedure of window settingapplied according to the embodiment.

First, it is assumed that the sequence of LLRs corresponding to the datasector and output first by the soft-decision most-likelihood decoder 18in response to read of data from the data sector carried out by the head12 is stored in the memory 210 in the LLR controller 21. The LLRcontroller 21 takes the absolute value (|LLR|) of each LLR in thesequence of LLRs (block 81). Then, based on the absolute value of eachLLR, the LLR controller 21 calculates the moving average of the absolutevalues of LLRs (block 82). Specifically, the LLR controller 21determines the average value of the absolute values of a certain numberof LLRs preceding and succeeding the target LLR to be the absolute valueof the target LLR. The LLR controller 21 iterates this operation on theabsolute values of all LLRs corresponding to the data sector tocalculate the moving average of absolute values of LLRs. In this manner,the moving average line 700 for the absolute values of LLRs shown inFIG. 7 is determined.

Then, based on the moving average line 700 and thresholds TH1 and TH2,the LLR controller 21 detects the range within which the value of themoving average line 700 is less than threshold TH1 and greater thanthreshold TH2, based on the rising portion and falling portion of themoving average line 700 (block 83). In the example in FIG. 7, the rangebetween P1 and P2 is detected based on the falling portion of the movingaverage line 700. The range between P3 and P4 is detected based on therising portion of the moving average line 700. In FIG. 7, P0 denotes thestart point (leading bit) of the data sector. Pn denotes the end point(final bit) of the data sector.

Based on the bit positions of P1, P2, P3, and P4, the LLR controller 21sets a window P0P1, a window P1P2, a window P2P3, a window P3P4, and awindow P4Pn (block 84). The range of window P0P1 is between P0 and P1.The range of window P1P2 is between P1 and P2. The range of window P2P3is between P2 and P3. The range of window P3P4 is between P3 and P4. Therange of window P4Pn is between P4 and Pn. Window P2P3 corresponds tothe defective portion. Windows P0P1 and P4Pn correspond to the normalportion. Windows P1P2 and P3P4 correspond to the boundary between thedefective portion and the normal portion. Whether windows P1P2 and P3P4are set to be the defective portion or the normal portion side may beselectively determined, for example, by the user.

In the above-described procedure of the window setting, it is assumedthat the windows are set based on the level classifying for the sequenceof LLRs corresponding to the data sector. However, the windows can alsobe set based on the level classifying for the amplitude (the absolutevalue of the amplitude) of the read signal corresponding to the datasector. In this case, the moving average of absolute value of theamplitude of the read signal may be used.

[Modification of Procedure of Window Setting]

Now, a modification of the procedure of window setting will be describedwith reference to the flowchart in FIG. 9. In the description below, toavoid complicatedness, the absolute values of LLRs are simply referredto as LLRs. The LLR controller 21 detects the point (bit position) P1with a predetermined number of consecutive bits, for example, fiveconsecutive bits, for LLRs less than threshold TH1 (block 91). Then, thecontroller 21 detects the point P2 which is located after P1 and whichinvolves a predetermined number of consecutive bits, for example, fiveconsecutive bits, for LLRs less than threshold TH2 (TH1>TH2) (block 92).

Then, the controller 21 detects the point P3 which is located after P2and which involves a predetermined number of consecutive bits, forexample, five consecutive bits, for LLRs greater than threshold TH2(block 93). The controller 21 then detects the point P4 which is locatedafter P3 and which involves a predetermined number of consecutive bits,for example, five consecutive bits, for LLRs greater than threshold TH1(block 94). Based on the bit positions of P1, P2, P3, and P4, the LLRcontroller 21 sets window P0P1, window P1P2, window P2P3, window P3P4,and window P4Pn as is the case with block 84 described above (block 95).

The procedure of window setting applied to the above-describedmodification, it is assumed that the windows are set based on the levelclassifying for the sequence of LLRs corresponding to the data sector.However, the windows can also be set based on the level classifying forthe amplitude of the read signal corresponding to the data sector.

Now, a method for determining the operation multiplier applied to eachwindow after shipment of HDD according to the embodiment will bedescribed with reference to FIG. 10. In FIG. 10, reference number 100denotes the moving average line of the absolute value of the amplitudeof the read signal corresponding to one data sector or the movingaverage line of the absolute value of LLRs to one data sector. Based onthe moving average line 100 and three thresholds TH1, TH2, and TH3(TH1>TH2>TH3), the LLR controller 21 sets windows W1, W2 _(L), W2 _(R),W3 _(L), W3 _(R), W4 _(L), and W4 _(R) are set for the sequence of LLRscorresponding to the data sector as shown in FIG. 10.

Here, window W1 corresponding to the defective portion is a first windowto which a predetermined operation multiplier (minimum multiplier orfirst multiplier), for example, 0.5, is applied. Windows W2 _(L) and W2_(R) corresponding to the normal portion are second windows to which apredetermined operation multiplier (maximum multiplier or secondmultiplier), for example, 2.0, is applied.

In contrast, each of windows W3 _(L) and W3 _(R) corresponding to thedefective portion side of the boundary between the defective portion andthe normal portion is a third window to which an operation multiplierless than 1 and greater than or equal to 0.5 is applied. The operationmultiplier applied to each of windows W3 _(L) and W3 _(R) is determinedbased on the inclination (more specifically, the absolute value of theinclination) of a segment of the moving average line 100 which islocated in the window as well as a predetermined function 102. Forexample, when the width of window W3 _(L) is defined as w3, theinclination of the moving average line 100 in window W3 _(L) isapproximated by the ratio of the difference between thresholds TH2 andTH3 to w3, that is, (TH2−TH3)/w3, and is 5/3 in the example in FIG. 10.If the function 102 is used, when the ratio is lower than a certainvalue, the operation multiplier decreases linearly with increasingratio. Then, when the ratio exceeds the certain value, the operationmultiplier is fixed to the minimum multiplier of 0.5.

Each of windows W4 _(L) and W4 _(R) corresponding to the normal portionside of the boundary between the defective portion and the normalportion is a fourth window to which an operation multiplier less than 1and greater than or equal to 2.0 is applied. The operation multiplierapplied to each of windows W4 _(L) and W4 _(R) is determined based onthe inclination (more specifically, the absolute value of theinclination) of a segment of the moving average line 100 which islocated in the window as well as a predetermined function 101. Forexample, when the width of window W4 _(L) is defined as w4, theinclination of the moving average line 100 in window W4 _(L) isapproximated by the ratio of the difference between thresholds TH1 andTH2 to w4, that is, (TH1−TH2)/w4, and is 5/4 in the example in FIG. 10.If the function 101 is used, when the ratio is lower than a certainvalue, the operation multiplier increases linearly with the ratio. Then,when the ratio exceeds the certain value, the operation multiplier isfixed to the maximum multiplier of 2.0.

Now, the method for determining the operation multiplier applied to eachwindow during the process of manufacturing HDD will be described withreference to FIG. 11. In FIG. 11, components equivalent to those in FIG.10 are denoted by the same reference numbers or characters. In FIG. 11,window W1 is a first window to which a predetermined operationmultiplier (maximum multiplier or first multiplier), for example, 2.0,is applied. Each of windows W2 _(L) and W2 _(R) is a second window towhich a predetermined operation multiplier (minimum multiplier or secondmultiplier), for example, 0.5, is applied.

In contrast, each of windows W3 _(L) and W3 _(R) is a third window towhich an operation multiplier greater than 1 and less than or equal to2.0 is applied. The operation multiplier applied to each of windows W3_(L) and W3 _(R) is determined based on the inclination of a segment ofthe moving average line 100 which is located in the window as well asthe predetermined function 101. For example, the inclination of themoving average line 100 in window W3 _(L) is approximated by the ratio(TH2−TH3)/w3 as described above, and is 5/3 in the example in FIG. 11.If the function 101 is used, when the ratio is lower than a certainvalue, the operation multiplier increases linearly with the ratio. Then,when the ratio exceeds the certain value, the operation multiplier isfixed to the maximum multiplier of 2.0.

Each of windows W4 _(L) and W4 _(R) is a fourth window to which anoperation multiplier less than 1 and greater than or equal to 0.5 isapplied. The operation multiplier applied to each of windows W4 _(L) andW4 _(R) is determined based on the inclination of a segment of themoving average line 100 which is located in the window as well as thepredetermined function 102. For example, when the width of window W4_(L) is defined as w4, the inclination of the moving average line 100 inwindow W4 _(L) is approximated by the ratio (TH1−TH2)/w4 as describedabove, and is 5/4 in the example in FIG. 11. If the function 102 isused, when the ratio is lower than a certain value, the operationmultiplier decreases linearly with increasing ratio. Then, when theratio exceeds the certain value, the operation multiplier is fixed tothe minimum multiplier of 0.5.

In the example in FIG. 11, the moving average line 100 includes asection in which the value is less than threshold TH3. Window W1 is setin association with this section. However, if the value of the movingaverage line 100 exceeds threshold TH3, window W1 is not set. The defectdetector 20 detects that no defective portion is present on thecorresponding data sector. If no defective portion is detected on thedata sector during the process of manufacturing HDD, then in theembodiment, the LLR operation mode is not set for the data sector.However, in response to the user's instruction, the LLR operation modemay be set depending on whether or not the moving average line 100includes a section in which the value is less than threshold TH1 or TH2.

In the embodiment, the LDPC code is applied as a parity check code.However, a single parity check code, a turbo code, or the like can beapplied as a parity check code. Furthermore, in the embodiment, it isassumed that the disk apparatus is HDD (magnetic disk drive). However,disk apparatuses other than HDD such as magneto-optical disk drives maybe used provided that iterative decoding is applied to the diskapparatus.

The various modules of the systems described herein can be implementedas software applications, hardware and/or software modules, orcomponents on one or more computers, such as servers. While the variousmodules are illustrated separately, they may share some or all of thesame underlying logic or code.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A method for operating log likelihood ratios in a disk apparatus towhich iterative decoding is applied, the method comprising: settingwindows for a sequence of the log likelihood ratios output by asoft-decision most-likelihood decoder based on the sequence of the loglikelihood ratios or an amplitude of a read signal acquired by a headconfigured to read data from a data sector on a disk; multiplying thelog likelihood ratios contained in each of the windows, by a multiplierspecific to each window; and transmitting the sequence of the loglikelihood ratios multiplied by the multiplier for each of the windowsto a parity check decoder.
 2. The method of claim 1 wherein the windowsare set based on the amplitude or the log likelihood ratios and at leastone threshold.
 3. The method of claim 2 wherein: the windows comprise afirst window corresponding to a defective portion on the data sector anda second window corresponding to a normal portion on the data sector; afirst multiplier is applied to the log likelihood ratios in the firstwindow, and a second multiplier is applied to the log likelihood ratiosin the second window; the second multiplier is greater than 1 if thefirst multiplier is less than 1; and the second multiplier is less than1 if the first multiplier is greater than
 1. 4. The method of claim 3wherein: the at least one threshold comprises a maximum threshold; andthe second window is set based on the amplitude or log likelihood ratiosand the maximum threshold.
 5. The method of claim 3 wherein: the atleast one threshold comprises a minimum threshold; and the first windowis set based on the amplitude or log likelihood ratios and the minimumthreshold.
 6. The method of claim 3 wherein after shipment of the diskapparatus, the first multiplier is less than 1, and the secondmultiplier is greater than
 1. 7. The method of claim 3 wherein duringmanufacturing of the disk apparatus, the first multiplier is greaterthan 1, and the second multiplier is less than
 1. 8. The method of claim3 wherein: the windows further comprise a third window and a fourthwindow corresponding to a boundary portion between the defective portionand the normal portion, the third window being set closer to the firstwindow, and the fourth window being set closer to the second window; athird multiplier is applied to the log likelihood ratios in the thirdwindow, and a fourth multiplier is applied to the log likelihood ratiosin the fourth window; the third multiplier is less than 1 and greaterthan or equal to the first multiplier and the fourth multiplier isgreater than 1 and less than or equal to the second multiplier if thefirst multiplier is less than 1; and the third multiplier is greaterthan 1 and less than or equal to the first multiplier and the fourthmultiplier is less than 1 and greater than or equal to the secondmultiplier if the first multiplier is greater than
 1. 9. The method ofclaim 8 wherein: the at least one threshold comprises a first threshold,a second threshold, and a third threshold, wherein the second thresholdis less than the first threshold, and the third threshold is less thanthe second threshold; the first window is set based on the amplitude orthe log likelihood ratios and the third threshold; the second window isset based on the amplitude or the log likelihood ratios, the secondthreshold, and the third threshold; the third window is set based on theamplitude or the log likelihood ratios, the first threshold, and thesecond threshold; and the fourth window is set based on the amplitude orthe log likelihood ratios and the first threshold.
 10. A disk apparatuscomprising: a soft-decision most-likelihood decoder configured to outputa sequence of log likelihood ratios from a bit sequence corresponding todata read from a data sector on a disk by a head, a parity check codeadded to the read data; a parity check decoder configured to decode datacorresponding to the sequence of the log likelihood ratios and toiterate an operation of updating the sequence of the log likelihoodratios based on the parity check code added to the data and transmittingthe updated sequence of the log likelihood ratios to the soft-decisionmost-likelihood decoder, in order to decode the data; a defect detectorconfigured to set windows for the sequence of the log likelihood ratiosoutput by the soft-decision most-likelihood decoder based on anamplitude of a read signal acquired in response to the read carried outby the head or the sequence of the log likelihood ratios; and a loglikelihood ratio controller configured to multiply the log likelihoodratios contained in each of the windows by a multiplier specific to eachwindow and transmit the log likelihood ratios multiplied by themultiplier for each of the windows to the parity check decoder.
 11. Thedisk apparatus of claim 10 wherein the defect detector is configured toset the windows based on the amplitude or the log likelihood ratios andat least one threshold.
 12. The disk apparatus of claim 11 wherein: thewindows comprise a first window corresponding to a defective portion onthe data sector and a second window corresponding to a normal portion onthe data sector; the log likelihood ratio controller is configured toapply a first multiplier to the log likelihood ratios in the firstwindow and to apply a second multiplier to the log likelihood ratios inthe second window; the second multiplier is greater than 1 if the firstmultiplier is less than 1; and the second multiplier is less than 1 ifthe first multiplier is greater than 1.